The invention relates to a process for coating substrates by means of cathode sputtering including a magnetron cathode with an annularly closed target which has a sputtering surface, an inner edge and an outer edge and a magnet system including permanent magnets. The one pole thereof which is geometrically similar to the target is disposed outside the outer edge and the other geometrically similar pole thereof, which is also annularly closed, is disposed inside the inner edge. They are disposed such that at least the greater part of the magnetic flux lines is guided over the sputtering surface on only slightly arcuate paths in a manner such that an annular magnetic tunnel is formed which is closed over the sputtering surface. With respect to ground potential as well as the cathode potential, the magnet system is at a self-adjusting electrical potential.
In a classical magnetron cathode as described in the paper "The Planar Magnetron" by Jon S. Chapin, published in Research/ Development, January 1974, the pole surfaces of the magnet system are behind the target and run parallel to the sputtering surface at least in the original state of the target. Moreover, the distances between the poles are relatively small. This magnetron construction leads to highly curved magnetic field lines which emerge from the sputtering surface and after passing through the arcuate paths, they reenter the target. The consequence is a relatively small and deep erosion trench allowing only a minor exploitation of the target material since the target must be replaced in time prior to being "sputtered through."
From DE-OS 22 43 708, it is also known to even out the consumption of the target by providing cylindrical and conical coils where the axes are disposed coaxially to the axes of correspondingly formed rotationally balanced targets. The advantages of such an arrangement, however, are effective only with hollow cathodes and post-like cathodes described therein. As far as planar cathode arrangements are concerned, the high degree of curvature of the magnetic flux lines again causes the undesired, locally limited and deep erosion trenches.
Attempts were also made to enlarge the erosion trench and the efficiency of the target, the so called material efficiency; U.S. Pat. No. 3,956,093 discloses superimposing the oscillating magnetic field of an electromagnet on the static magnetic field of the permanent magnet so that the race-track-like, closed magnetic tunnel is periodically shifted on the target surface. The plasma enclosed in the magnetic tunnel follows this periodic shift thus enlarging the erosion trench. The high degree of curvature of the flux lines of the permanent magnet (the curvature corresponds to approximately the one described in the paper by Chapin), however, requires extremely high field strengths of the magnetic coil in order to produce a noticeable shift of the superimposed magnetic field. Moreover, due to the constant pole reversal of the electromagnet, the magnetic field is only shifted starting from a center position. This center position is prescribed by the course of the static magnetic flux lines of the permanent magnet. A consequence thereof is that the magnetic coil consumes a significant amount of current which in turn leads to cooling problems.
From DE-OS 27 29 286 (GB-A 1,587,566) it is also known to mechanically shift a permanent magnetic field. This however, involves a great amount of technical labor.
Hence, other attempts were also made leading to the above described process. A corresponding magnetron cathode, is known from U.S. Pat. No. 4,515,675. In this magnetron cathode, only the substantially less arcuate portion of the magnetic flux lines concentrates the plasma onto the surface of the cathode. This is achieved in that the poles of the magnetic system enclose the target cross section, i.e. the target has an annular configuration and an opening in its center where the center pole of the magnetic system is disposed. It is thus possible to achieve a uniform wear of the target without employing an additional oscillating magnetic field. However, achieving this uniformity is still not sufficient, especially when the target plates have a greater thickness, for example, more than 15 mm.
It is common to all of the above described magnetron cathodes that the plasma which causes the sputtering is more or less highly concentrated toward the direct vicinity of the target surface, the so called sputter surface. The present solutions, neither with or without the use of oscillating magnetic fields, permit an expansion of the plasma in direction of the substrates which would be desirable for certain coating processes.
On the other hand, it is also known to further expand the plasma in direction to the substrates by selectively "detuning" the known magnetron cathode and, for example, to force the flux lines emanating from the outer magnetic poles to follow a longer path, the latter extending to the substrates. A measure of this kind is described by Window and Savvides in the paper "Charged particle fluxes from planar magnetron sputtering sources," published in J. Vac. Sci. Technol. A 4 (2), March/April 1986. However, the problem of achieving a uniform wear of the target is not solved.
Such detuned magnetrons are also referred to as "unbalanced magnetrons." For further details refer to the paper by Biederman et. al. "Hard Carbon and composite metal/hard carbon films prepared by a DC unbalanced planar magnetron" (published in "Vacuum", issue 40, no. 3, 1990, pages 251 to 255).
The use of the above described and known magnetron cathodes for reactive cathode sputtering, i.e. the application of chemical compounds, where the target material (usually a metal) is only one component poses further problems: Usually, the actual sputter gas used is a noble gas, predominantly argon. A corresponding reactive gas is added to this noble gas. When oxides are produced, the reactive gas is oxygen; when nitrides are produced, it is nitrogen; when carbides are produced, it is carburetted hydrogen gas, and so on. Generally, the reaction products are electrically non-conductive and also partially form on the target surface. In the practice, this cannot be completely excluded. It is only in the area of the erosion trench that the material is sputtered faster than the reaction products can form. In the remaining surface areas, however, electrical charges can build up which finally lead to voltage arcovers followed by an instable sputtering process, not least since the controlled power supply to the sputtering cathode naturally responds to these voltage arcovers.